This invention relates to a process for the removal of organic sulfur compounds by oxidation from hydrocarbon fuels which have relatively low amounts of sulfur present, such as in fuels which have been through a hydrogenation step to remove organic sulfur compounds.
The presence of sulfur in hydrocarbons has long been a significant problem from the exploration, production, transportation, and refining all the way to the consumption of hydrocarbons as a fuel, especially to power automobiles and trucks. Now, it has become an environmental objective to rid fuels such as diesel fuel, gasoline, fuel oils, jet fuel, kerosine, and the like of the troublesome residual organic sulfur present in such hydrocarbons, even though on a relative basis the amount present to begin with is small such as, for example, in diesel fuel the sulfur content may be about 500 parts per million by weight, or less. However, under the present regime even this amount has become too much with extant and prospective regulation of sulfur emissions from many sources becoming increasingly strict.
The prior art is replete with attempts to reduce the sulfur content of hydrocarbon by both reduction and oxidation of organic sulfur present. Much of this prior art relating to oxidation has taught the use of various peroxides in conjunction with a carboxylic acid and, specifically, the preferred species involved in the practice of this invention; i.e., hydrogen peroxide and formic acid. For example, U.S. Pat. No. 5,310,479 teaches the use of formic acid and hydrogen peroxide to oxidize sulfur compounds in crude oil, limiting the application of the technology only to aliphatic sulfur compounds. There was no hint of the removal of aromatic sulfur compounds. This patent discussion is directed to the removal of sulfur from crude oil rich (about 1-4%) in sulfur compounds. The acid to peroxide ratio was indiscriminately broad and failed to recognize the economic disadvantages to using hydrogen peroxide in attempts to remove large amounts of sulfur, while at the same time failing to recognize the importance of controlling the presence of water to the successful operation. Water was used to extract the sulfones from the treated hydrocarbon in a separate wash step. Further, the prior art also fails to recognize the beneficial effect of limiting the peroxide concentration to low values without compromising either the rate or extent of oxidation of the sulfur compounds.
A recent study entitled xe2x80x9cOxidated Desulfurization of Oils by Hydrogen Peroxide and Heteropolyanion Catalyst,xe2x80x9d Collins, et al., published Journal of Molecular Catalysis A: Chemical, 117 (1997) 397-403, discusses other studies to oxidatively remove sulfur from fuel oil, but large quantities of hydrogen peroxide were required. However, the experimental work did show that unacceptable amounts of hydrogen peroxide were consumed thus suggesting the cost of oxidative reduction of sulfur in feedstocks for diesel fuel to be impermissibly high.
In European Patent Application Publication No. 0565324A1, a method for recovering organic sulfur compounds from liquid oil is described. While the stated objective of the patent publication is to recover the organic sulfur compound, the treatment involves using a mixture of a number of oxidants, one of which is disclosed as a mixture of formic acid and peroxide. The distillation products, the organic sulfones, are removed by a number of methods including absorption on alumina or silica adsorbent materials. The treatments described are characterized by use of a low ratio of formic acid to the hydrogen peroxide.
While this and other prior art recognize the reaction kinetics and mechanism of hydrogen peroxide and other peroxides with organic sulfur compounds present in various fuels, none recognize the combination of factors necessary to successfully and economically remove relatively small amounts of sulfur present in fuels such as diesel oil, kerosine, gasoline, and light oils down to residual levels approaching zero. While low amounts of sulfur will be construed to mean in the context of this invention, those amounts which are less than about 1500 parts per million, an eaxample demonstrates effective removal of 7000 ppm of sulfur such that the present invention is applicable to higher levels of sulfur. Of course in some instances, the practice of this invention may be economically and technically applicable to the treatment of fuels having a sulfur content at these elevated levels. It has been found in the practice of this invention that the sulfur content of the fuel which is left unoxidized is less than about 10 ppm of sulfur, often as low as between 2 ppm and 8 ppm. Oxidation alone does not necessarily ensure total removal of the sulfur to the same low residual sulfur values since some of the oxidized sulfur species do have a non-zero solubility in the fuel, and a partition coefficient that defines their distribution in the oil phase in contact with a substantially immiscible solvent phase, whether it is an organic solvent as in prior art, or the high acid aqueous phase of this invention. In addition to the substantially complete and rapid oxidation of the relatively low amounts of sulfur in the fuel feed, the present invention also teaches the substantially complete removal of the oxidized sulfur to residual levels approaching zero, and the recovery of the oxidized sulfur compounds in a form suitable for their practical further disposition in an environmentally benign way.
The sulfur compounds which are most difficult to remove by hydrogenation appear to be the thiophene compounds, especially benzothiopene, dibenzothiopene, and other homologs. In an article, Desulfurization by Selective Oxidation and Extraction of Sulfur-Containing Compounds to Economically Achieve Ultra-Low Proposed Diesel Fuel Sulfur Requirements (Chapados, et al., NPRA Presentation, Mar. 26-28, 2000) the oxidation step involved the reaction of the sulfur in a model compound using dibenzothiophene with a peroxyacetic acid catalyst made from acetic acid and hydrogen peroxide. The reaction with the peroxyacid was conducted at less than 100xc2x0 C. at atmospheric pressure and in less than 25 minutes. After extraction, the process resulted in a reduction of the sulfur content in the diesel fuel. Still, the cost was indicated to be high with the hydrogen peroxide being the biggest cost item and consumed in the process due in large part to the lack of recognition of the part excessive water plays in the efficient utilization of low amounts of hydrogen peroxide.
It has been discovered that fuel oils such as diesel fuel, kerosene, and jet fuel, though meeting the present requirements of about 500 ppm maximum sulfur content, can be economically treated to reduce the sulfur content to an amount of from about 5 to about 15 ppm, in some instances even less. In practicing the process of the present invention the hydrocarbon fuel containing low amounts of organic sulfur compounds, i.e., up to about 1500 ppm, is treated by contacting the sulfur-containing fuel with an oxidizing solution containing hydrogen peroxide, formic acid, and a limit of a maximum of about 25 percent water. The amount of the hydrogen peroxide in the oxidizing solution is greater than about two times the stochiometric amount of peroxide necessary to react with the sulfur in the fuel. The oxidizing solution used contains hydrogen peroxide at low concentration, the concentration, in its broadest sense, being from about 0.5 wt % to about 4 wt %. The reaction is carried out at a temperature of from about 50xc2x0 C. to about 130xc2x0 C. for less than about 15 minutes contact time at close to, or slightly higher than atmospheric pressure at optimum conditions. The oxidizing solution of the invention has, not only a low amount of water, but small amounts of hydrogen peroxide with the acid, with the formic acid being the largest constituent. The oxidation products, usually the corresponding organic sulfones, become soluble in the oxidizing solution and, therefore, may be removed from the desulfurized fuel by an almost simple simultaneous extraction and a subsequent phase separation step. The aqueous phase is removed from the hydrocarbon phase which now has a reduced sulfur content. While all sulfur-containing constituents of the fuel may not be removed to the desired very low residual sulfur levels by the extraction step into the now spent oxidizer solution, the conversion and concentration reduction of sulfur in such fuels in the oxidation step provide a more easily accomplished extraction and removal to almost completely desulfurize the resulting liquid hydrocarbons; such as fuel oils, diesel fuel, jet fuel, gasoline, coal liquids, and the like to levels of about 5 to 15 ppm sulfur, and often approaching zero. Where there is a residual amount of oxidized sulfur compounds, usually sulfones, in the fuel, this invention enables the practical and economic use of additional separation steps to remove the residual sulfur by selected solid adsorbants such as, for example, in a cyclic adsorption-desorption operation to achieve a sulfur-free fuel product, and recover the oxidized sulfur compounds in a concentrated form and in a way practical for their final environmentally benign, disposition within a refinery.
Once the extract containing the oxidized sulfur compounds is separated from the desulfurized fuel, or raffinate, the extract can be treated to recover the acid for recycle. The separation is accomplished in a number of ways, but the preferred separation occurs by the use of a liquid-liquid separator operated at a temperature sufficiently high, close to the oxidation reaction temperature, to result in gravity separation of the material without appearance of a third, precipitated solid phase. The aqueous phase, of course, being heavier than the oil phase would be drained from the bottom of the separation device where it may be preferably mixed with a suitable high boiling range refinery stream, such as for example, a gasoil, and flash distilled to remove the water and acid overhead while transferring and leaving the sulfur-containing compounds into the gasoil stream exiting at the bottom of the distillation column. The overhead stream containing acid and water from the flash distillation and sulfone transfer column is further distilled in a separate column to remove portion of its water for disposal. The acid recovered can then be returned to the oxidizing solution make up tank where it is combined with the hydrogen peroxide to form the oxidizing solution and again contact the sulfur-containing fuel feed. This preservation of the acid enhances the economics of the process of this invention.
After separation the fuel maybe further heated and flashed to remove any residual acid/water azeotrope, which can be recycled to the liquid-liquid separation step, or elsewhere in the process. Then the fuel may be contacted with a caustic solution, or with anhydrous calcium oxide (i.e., quicklime) and/or passed through filtering devices to neutralize any trace acid remaining and to make a final dehydration of the fuel. The fuel stream may be then passed over a solid alumina bed, at ambient temperature, to adsorb the residual oxidized sulfur compounds soluble in fuel, if any are present. The product is now thoroughly desulfurized, neutralized, and dry.
The oxidized sulfur compounds adsorbed on alumina may be removed by desorption and solubilization into a suitable hot polar solvent, methanol being the preferred solvent. Other suitable solvents are acetone, THF (tetrahydrofuran), acetonitrile, chlorinated solvents such as methylene chloride as well as the aqueous oxidizer solution with high acid contents of this invention. One advantage of the adsorption/desorption system of this invention is that it can use commercially-available alumina adsorbants that are used in multiple cycles without significant loss of activity and without the need to reactivate them by conventionally employed high temperature treatment for dehydration. The extracted oxidized sulfur compounds are transferred into higher boiling refinery streams for further disposition by flash distillation, which also recovers the methanol for recycle in the alumina desorption operation.
The oxidizing solution of the invention is preferably formed by mixing a commercially-available 96%, by weight, formic acid solution with a commercially-available hydrogen peroxide solution, normally the 30%, 35% and 50 wt % concentration commercially available in order to avoid the dangers connected with handling a 70% hydrogen peroxide solution in a refinery environment. The solutions are mixed to result in an oxidizing material containing from about 0.5 to about 4 wt % hydrogen peroxide, less than 25 wt % water with the balance being formic acid. The water in the oxidizer/extractor solution normally comes from two sources, the dilution water in the peroxide and acid solutions used, and the water in the recycled formic acid, when the process operates in the recycle mode. On occasion, additional water could be added without being detrimental to the practice of this invention as long as the criteria explained herein are considered, but it is important to an economical process to keep the water content low as set forth herein. The preferable concentration of hydrogen peroxide, which is consumed in the reaction, in the oxidizer solution would be from about 1% to about 3% by weight, and most preferably from 2 to 3 wt %. The water content would be limited to less than about 25 wt %, but preferably between about 8 and about 20%, and most preferably from about 8 to about 14 wt %. The oxidation/extraction solution used in the practice of this invention will contain from about 75 wt % to about 92 wt % of carboxylic acid, preferably formic acid, and preferably 79 wt % to about 89 wt % formic acid. The molar ratio of acid, preferably formic acid, to hydrogen peroxide useful in the practice of this invention is at least about 11 to 1 and from about 12 to 1 to about 70 to 1 in the broad sense, preferably from about 20 to 1 to about 60 to 1.
This will accomplish a rapid and complete oxidation of the sulfur compounds, and their substantial extraction from such refined products as diesel fuel, jet fuel, or gasoline which contain from about 200 to about 1500 ppm sulfur and will perform effectively to oxidize and extract organic sulfur present in fuels at greater concentrations. Since the moles of hydrogen peroxide to be used is proportional to the amount of sulfur present and since the peroxide is consumed, the cost of this material can have a negative effect on the economics of the operation if the amount of sulfur present is excessive or if there are other hydrocarbons present in the material being treated which will be oxidized, such as, for example, in crude oil. Of course hydrogen peroxide has a natural tendency to decompose to water and non-reaction oxygen under these conditions. Therefore, this invention is properly most useful for polishing small amounts of sulfur, such as for example less than about 1000 ppm, from hydrocarbon fuels ready for market than for removal of sulfur from crude oil containing gross amounts of sulfur.
In the oxidation of organic sulfur compounds using hydrogen peroxide, the stochiometric reaction ratio is two moles of the hydrogen peroxide consumed per mole of sulfur reacted. In the practice of this invention the amount of oxidizing solution used should be such that it contains at least about two times the stochiometric amount to react the sulfur present in the fuel, preferably from about two to about four times. Greater amounts could be used, but only at increased cost since it has been found that improvement of sulfur oxidation is marginal at best when the amount is greater than four times the amount needed. Furthermore, to minimize peroxide losses by decomposition side reactions, the hydrogen peroxide concentrations in the oxidizer composition of this invention are preferably adjusted at low levels about 0.5 wt % to about 4 wt %. At these levels and the reaction temperature of about 95xc2x0 C., it has been surprisingly discovered that the rapid and complete oxidation, and extraction, of the sulfur compounds from hydrocarbon feeds of relatively low sulfur content, compete favorably with the side reaction of peroxide decomposition, resulting in a practical and economic process for desulfurization of such fuels. Normally, the sulfur present would be calculated on the basis of it being a thiophenic sulfur. If the sulfur originally contained in the fuel is all dibenzothiophene or thiophene sulfur, then the removal from the oxidation/extraction step can result in less than about 10 ppm sulfur in the treated fuel. Other sulfur-containing compounds could, even though oxidized, cause additional extraction and removal steps to be performed depending upon the type of sulfur involved and the solubility in the fuel being treated.
Surprisingly, by limiting the water and hydrogen peroxide present and the reaction conditions of this invention, a practical process results with almost complete oxidation of organic sulfur compounds at high rates, with low peroxide concentrations, at relatively small peroxide excess over the stoichiometric requirement, and on feeds with relatively low sulfur content; all of these conditions being recognized in the art as kinetically unfavorable conditions. In addition to this unexpected result, it is accomplished with little loss of the expensive hydrogen peroxide to expected side reactions of self decomposition, or with other hydrocarbon species.
While the following invention is described in some detail, it must be understood by those skilled in the art that there is no intention on the part of the inventors hereof to abandon any part of the concepts of this invention with respect to the reduction of the organic sulfur in fuels and light oils.